Catalytic reforming, or hydroforming, is a well-established industrial process employed by the petroleum industry for improving the octane quality of naphthas or straight run gasolines. In reforming, a multi-functional catalyst is employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, notably alumina. Noble metal catalysts, notably of the platinum type, are currently employed in reforming. Platinum has been widely commercially used in recent years in the production of reforming catalysts, and platinum-on-alumina catalysts have been commercially employed in refineries for the last few decades. In the last decade, additional metallic components have been added to platinum as promoters to further improve the activity or selectivity, or both, of the basic platinum catalyst, e.g., iridium, rhenium, tin, and the like. Reforming is defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of normal paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst.
In a conventional process, a series of reactors constitute the heart of the reforming unit. Each reforming reactor is generally provided with fixed beds of the catalyst which receive upflow or downflow feed, and each is provided with a heater, because the reactions which take place are endothermic. A naphtha feed, with hydrogen, or hydrogen recycle gas, is concurrently passed through a preheat furnace and reactor, and then in sequence through subsequent interstage heaters and reactors of the series. The product from the last reactor is separated into a liquid fraction, and a vaporous effluent. The latter is a gas rich in hydrogen, and usually contains small amounts of normally gaseous hydrocarbons, from which hydrogen is separated from the C.sub.5.sup.+ liquid product and recycled to the process to minimize coke production.
The activity of the catalyst gradually declines due to the buildup of coke. Coke formation is believed to result from the deposition of coke precursors such as anthracene, coronene, ovalene and other condensed ring aromatic molecules on the catalyst, these polymerizing to form coke. During operation, the temperature of the process is gradually raised to compensate for the activity loss caused by the coke deposition. Eventually, however, economics dictates the necessity of reactivating the catalyst. Consequently, in all processes of this type the catalyst must necessarily be periodically regenerated by burning the coke off the catalyst at controlled conditions, this constituting an initial phase of catalyst reactivation.
Two major types of reforming are generally practiced in the multi-reactor units, both of which necessitate periodic reactivation of the catalyst, the initial sequence of which requires regeneration, i.e., burning the coke from the catalyst. Reactivation of the catalyst is then completed in a sequence of steps wherein the agglomerated metal hydrogenation-dehydrogenation components are atomically redispersed. In the semi-regenerative process, a process of the first type, the entire unit is operated by gradually and progressively increasing the temperature to maintain the activity of the catalyst caused by the coke deposition, until finally the entire unit is shut down for regeneration, and reactivation, of the catalyst. In the second, or cyclic type of process, the reactors are individually isolated, or in effect swung out of line by various manifolding arrangements, motor operated valving and the like. The catalyst is regenerated to remove the coke deposits, and reactivated while the other reactors of the series remain on stream. A "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, until it is put back in series.
There are several steps required for the regeneration, and reactivation of a catalyst. Typically, regeneration of a catalyst is accomplished in a primary and secondary coke burnoff. This is accomplished, initially, by burning the coke from the catalyst at a relatively low temperature, i.e., at about 800.degree. F.-950.degree. F., by the addition of a gas, usually nitrogen or flue gas, which contains about 0.6 mole percent oxygen. A characteristic of the primary burn is that essentially all of the oxygen is consumed, with essentially no oxygen being contained in the reactor gas outlet. Regeneration is carried out once-through, or by recycle of the gas to the unit. The temperature is gradually raised and maintained at about 950.degree. F. until essentially all of the coke has been burned from the catalyst, and then the oxygen concentration in the gas is increased, generally to about 6 mole percent. The main purpose of the secondary burn is to insure thorough removal of coke from the catalyst within all portions of the reactor. The catalyst is then rejuvenated with chlorine and oxygen, reduced, and then sulfided. Thus, the agglomerated metal, or metals, of the catalyst, is redispersed by contacting the catalyst with a gaseous admixture containing a sufficient amount of a chloride, e.g., carbon tetrachloride, to decompose in situ and deposit about 0.1 to about 1.5 wt.% chloride on the catalyst; continuing to add a gaseous mixture containing about 6% oxygen for a period of 2 to 4 hours while maintaining temperature of about 950.degree. F.; purging with nitrogen to remove essentially all traces of oxygen from the reactor; reducing the metals of the catalyst of contact with a hydrogen-containing gas at about 850.degree. F.; and then sulfiding the catalyst by direct contact with, e.g., a gaseous admixture of n-butyl mercaptan in hydrogen, sufficient to deposit the desired amount of sulfur on the catalyst. The primary coke burnoff step is extremely time-consuming, the primary coke burn frequently accounting for up to one-half of the time a reactor is off-oil for regeneration, and reactivation; and, a major consideration in the regeneration/reactivation sequence relates to the rate at which oxygen can be fed into a reactor. The total heat released is directly proportional to the amount of coke burned, and hence the rate at which oxygen can be fed into the reactor then is governed by the rate at which heat can be removed from a catalyst bed, and reactor, so that the flame front temperature in a bed does not become sufficiently overheated to damage the catalyst. Generally, it is desired that the regeneration temperature not exceed about 950.degree. F. to about 975.degree. F.
It is, accordingly, a primary objective of the present invention to shorten the time required for regeneration of noble metal reforming catalysts, especially platinum-containing reforming catalysts.
A specific object is to provide a novel process for the regeneration of such catalysts, especially as relates to the use of such catalysts in cyclic reforming units, notably one which will shorten the time required for regeneration of such catalysts; this permitting an increase in regeneration frequency so that all reactors can operate at a fresher level of catalyst performance to provide increased overall catalyst activity and increased C.sub.5.sup.+ liquid yields.
A further, and more specific object is to provide a process which will lower compression costs by reducing the amount of gas that must be compressed and injected into a reforming unit during catalyst regeneration.
These objects and others are achieved in accordance with the present invention, embodying improvements in a process for regenerating, and reactivating, noble metal catalysts, especially platinum-containing polymetallic catalysts, by the use of a gas for burning coke from a coked catalyst comprising an admixture of from about 0.1 percent to about 10 percent oxygen, preferably from about 0.2 percent to about 7 percent oxygen, and more preferably from about 0.2 to about 4 percent oxygen, and at least about 20 percent carbon dioxide, preferably from about 40 percent to about 99 percent, and more preferably from about 50 percent to about 99 percent carbon dioxide, based on the total volume of the regeneration gas. Water, or moisture levels are maintained below about 5 volume percent, preferably below about 2 volume percent during the burn. In accordance with this invention, albeit carbon dioxide does not participate in the reaction to any appreciable extent, if any, it has been found that regeneration time can be considerably shortened, the frequency of reactor regeneration increased, and compression costs lowered by increasing, or maximizing, the carbon dioxide content of the gas used in the coke burnoff, particularly that portion of the regeneration period referred to as the primary coke burnoff. The higher heat capacity of the carbon dioxide permits the use of a greater amount of oxygen in the regeneration gas which is fed to a reactor and contacted with a catalyst, particularly during the primary coke burn, as contrasted with the regeneration gas used in conventional catalyst regeneration processes which contain large amounts of nitrogen and flue gas as inert gases.
Over a temperature range of 800.degree. F. to 980.degree. F., e.g., carbon dioxide has an average heat capacity 63 percent greater than that of nitrogen (12.1 Btu/lb mole -.degree.F. for CO.sub.2 versus 7.43 Btu/lb mole -.degree.F. for nitrogen). Therefore, for a reactor inlet gas temperature of about 750.degree.-800.degree. F. and a flame front temperature of about 950.degree.-975.degree. F., carbon dioxide will absorb roughly 63 percent more heat than an equivalent volume of nitrogen at corresponding temperatures. For the two extreme cases where the non-oxygen portion of the oxygen-containing gas which is fed to the reactor in which the coke is being burned consists almost entirely of either carbon dioxide, or of nitrogen, the concentration of oxygen at the reactor inlet can be about 63 percent greater in the case of complete carbon dioxide. This can reduce that the total catalyst burn time by nearly 40 percent. It is found that the substitution of carbon dioxide for flue gas in a conventional catalyst regeneration gas can achieve a 25 percent reduction in the time required for the primary burn. The further substitution of oxygen for air in addition to the substitution of carbon dioxide for flue gas can provide a full 33 percent reduction in primary burn time. In each case, compression costs are lowered because of the reduced volume of gas involved per pound of coke burned.
Average catalyst activities, and overall C.sub.5.sup.+ liquid yields are improved, especially in regenerating the catalyst in cyclic reforming units, vis-a-vis the regeneration of catalysts in conventional regeneration units, by maximizing the carbon dioxide content (specifically, the CO.sub.2 /NO.sub.2 ratio) of the gas circulation system during the coke burnoff phases of catalyst regeneration, particularly during the primary burn. The higher heat capacity of carbon dioxide permits a higher concentration of oxygen in the regeneration gas which is fed to the reactor. Regeneration times are consequently shortened and the frequency of reactor regeneration is increased. Catalyst activity and yields are improved. In addition, compression costs are lower than those of conventional nitrogen or flue gas regeneration systems.
These features and others will be better understood by reference to the following more detailed description of the invention, and to the drawings to which reference is made.